To an increasing extent, digital equipment is being used for the transmission of information, Moreover, data transmission is gaining increasing significance. To meet anticipated future requirements, therefore, a communication network is needed which is equally well suited to switching different types of information. The services to be integrated in such a network differ in various ways. For example, one can distinguish between services where a fairly steady information flow exists during relatively short periods of time, and services where information flows briefly from time to time over prolonged periods of time. There are two different switching methods adapted to those requirements. A switching method in which a direct transmission path between the terminals involved is made available for the duration of a call, regardless of whether information is transmitted or not, is called "circuit switching". A switching method in which the messages are divided into packets and routed through the network link by link with the aid of destination information contained in a packet header is called "packet switching". The packets are stored until a path in a desired direction becomes free. There is no through-switching of transmission paths. A special case is store-and-forward switching, where undivided messages are routed through the data network link by link from switching center to switching center, e.g., in electronic mail systems. More formal definitions are contained in "NTG-Empfehlung 0902", 1982, items 4.2.1 and 4.2.2 (published in "ntz," No. 8/82, page 549), which are hereby incorporated by referenced.
Obviously the switching equipment must be comparable with the particular switching method utilized.
One way of performing both circuit switching and packet switching is described in an article by A. Chalet and R. Drignath, "Datenmodul-Architektur mit Paketverarbeitungsfunktionen," Elektrisches Nachrichtenwesen, Vol. 59, No. 1/2, 1985. In that system, packet switching is effected by storing each packet in a packet memory at the input of the switching center, then setting up a path through the switching network like in a circuit-switching system, then transmitting the packet over this path, and finally clearing this path. To accelerate the setting up of a path for each packet, the parameters necessary for call setup (in particular, the destination address) are determined in a call setup phase and are stored so as to be immediately available upon arrival of a packet.
A completely different proposal was presented by John J. Kulzer and Warren A. Montgomery at the ISS '84 Florence, 7-11 May 1984, in a paper entitled "Statistical Switching Architectures for Future Services" (Session 43 A Paper 1). A similar proposal was submitted at the same conference by A. Thomas et al., "Asynchronous Time-Division Techniques: an Experimental Packet Network Integrating Videocommunication" (Session 32 C Paper 2). According to those proposals, all information, from sporadically occurring single instructions to digitized video signals, is divided into packets and passed on by packet switching. According to FIG. 6 of the Kulzer article and the pertinent description, the individual packets are routed through an exchange from stage to stage, where they are temporarily stored as required.
Both prior art solutions outlined above have advantages and disadvantages. If signals have to be switched at high transmission speeds (so-called broadband switching), some of those advantages and disadvantages are particularly significant.